Fuel cells are comprised of electrochemical cells used for providing an environmentally clean method for generating electricity. What makes fuel cells different from another electrochemical energy converter, such as a battery, is the fact that both fuel and oxidant are continuously supplied to their respective electrodes, and reaction products are continuously removed from the fuel cell. Electric current will continue to flow essentially as long as fuel and oxidant are supplied to the electrodes. Fuel cell systems can be formed by stacking and electrically connecting many electrochemical cells together to provide power generation for residential, commercial and industrial scale power applications. Individual fuel cells in fuel cell systems each include at least two catalytic electrodes in contact with an electrolyte medium comprising an electrode-electrolyte assembly. The individual fuel cells also include devices for managing fuel and oxidant flows thereto and for controlling temperature within operating limits. Use of pure hydrogen as a fuel results in higher fuel cell energy density outputs compared to other fuels. However, hydrogen has a number of drawbacks including: flammability; storage difficulties; and comparatively high production costs.
In addition to hydrogen, naturally occurring organic fuels and synthetic fuels can be used in fuel cells. Naturally occurring fuels are preferred over synthetic fuels because of their abundance and lower cost compared to cost prohibitive synthetic fuels. Naturally occurring organic fuels as well as synthetic fuels can form hydrogen external to the fuel cell system using an endothermic chemical reaction such as steam reforming. However, steam reforming is a slow responding process because it relies on thermal energy input to accommodate load changes. As such, steam reforming is limited mostly to steady state fuel cell operations at temperatures much higher than ambient temperature. The steady state operating limitation makes such fuel cells impractical for varying power output to follow transient electric load demands. Moreover, operation of fuel cells at such high temperatures precludes the use of most polymer electrolyte membranes. Various fuel cell designs have utilized steam reformers external to the fuel to allow for fuel cell operation at ambient temperatures. However, steam reforming outside a fuel cell increases cost and does not provide improved transient load following capability. Hydrogen generated by steam reformers external to the fuel cell could be accumulated in a storage facility. However, storage of highly flammable fuels such as hydrogen is dangerous. Moreover hydrogen storage facilities generally limit fuel cells to stationary applications.
Modifications of fuel cell electrodes to utilize hydrogen from naturally occurring organic matter include use of ruthenium in the catalyst on the electrodes, which can lower operating temperature requirements below the boiling point of water. However, fuel cells comprising ruthenium containing catalytic electrodes are typically operated above ambient temperature.
Hydrogen can be obtained at ambient temperature (i.e., without steam reforming) from simple forms of water-soluble organic fuels such as methanol. However, use of methanol is generally not cost-effective enough for widespread application. Use of complex organic fuels, such as hexose, is desirable for use in fuel cells because of their natural abundance and competitive cost. When used in fuel cells, complex organic fuels such as hexose react to release hydrogen in a sequence of electrochemical de-hydrogenation reactions. Typically, intermediates are produced as a result of such de-hydrogenation reactions. These intermediates are further reacted to waste products. Some of these intermediates, however, are known to poison and deactivate the fuel side of catalytic electrodes, essentially stopping the production of hydrogen. Certain fuels such as methanol are less likely to cause fuel side catalytic electrode poisoning if operated at elevated temperatures. However, methanol has a high permeability through electrolyte membranes and can diffuse through the membrane thereby polarizing the oxidant side of the catalytic electrode. Such polarization reduces the performance of the fuel cell.
Hydrogen permeable metal barriers have been used to limit the diffusion of methanol across electrolyte membranes. However, use of metal barriers also limits the transport of electrochemically active species such as hydrogen ions and neutral atoms and thus, limits the performance achievable directly from methanol fuel. The approach where the access of methanol to the electrode is controlled by means of other permeable membranes, such as polymers, has the disadvantage of requiring elevated temperature for proper operation and to exceed the performance levels of fuel cells having metal barriers.
The performance of fuel cells using catalytic electrodes can degrade due to catalyst deactivation and poisoning by reaction intermediates, especially near ambient operating temperature. For catalytic electrodes comprising platinum, carbon monoxide is a likely poisoning intermediate. Elevation of the operating temperature of the fuel cell to about 200° C. can eliminate such poisoning. While elevating the operating temperature of the fuel cell may be practical in fuel cell applications operating continuously at or near steady state, it is difficult to implement for applications that use the fuel cell on a transient or as-needed basis and makes the use of polymer electrolyte assemblies impractical.
There is a need to provide a fuel cell system including a fuel processing device/system capable of processing complex fuels internal to the fuel cell at near ambient temperature. Prior art methods and systems for addressing these needs for portable or transient applications were either too expensive, inefficient, or ineffective or a combination of all of these. Based on the foregoing, it is the general object of the present invention to improve upon or overcome the problems and drawbacks of the prior art.